Amazing facts about the human blood

We all know blood as that red fluid that comes out of our body when we have a cut or an injury. But what do we really know about our blood?

The human blood is a highly specialized, constantly circulating tissue that delivers oxygen and nutrients to cells and transports wastes such as ammonia and carbon dioxide away from those cells. Blood is conducted through blood vessels (arteries, veins, capillaries and lymphatics). It is about 7% of your overall body weight and your body carries about 6 litres of blood. Not all blood is red. Some animals like crabs have blue blood. The four most important components of the human blood are red blood cells (erythrocytes), white blood cells (leukocytes), platelets (thrombocytes) and plasma.

The red blood cells normally make up 40-50% of total blood volume. They transport oxygen from the lungs to all the living tissues of the body and carry away CO2 (carbon dioxide). These cells are produced continuously in our bone marrow from stem cells. Red cells remain viable for only about 4 months before they are removed from the blood and their components recycled in the spleen. People that are anaemic generally have a deficiency in red cells and subsequently feel fatigued due to shortage of oxygen.

White blood cells vary in types and numbers. It makes up about 1% of blood volume in healthy people. Most are produced in our bone marrow from the same kind of stem cells that produce the red blood cells and they are usually active in mounting immune responses to infectious diseases and other foreign invaders.

Blood platelets are cell fragments that work with blood clotting chemicals at the site of wounds. They do this by adhering to the walls of blood vessels and releasing coagulating chemicals that cause clots to form. 13 different blood clotting factors, in addition to platelets, need to interact for clotting to occur.

Plasma is a relatively clear yellow tinted liquid. It is composed of water (about 90%), fat, protein and salt solution which carry the red cells, white cells and platelets. Normally 55% of our blood’s volume is made up of plasma. Plasma also contains blood clotting factors, sugars, lipids, vitamins, minerals, hormones, enzymes, antibodies and other proteins. Some of the molecules found in the plasma have more specialized functions e.g. hormones act as long distance signals.

Although all human blood is red and have relatively the same composition, not all human blood is actually the same. There are different types of blood and different blood grouping systems. The most important blood grouping system is the ABO system, which was discovered in 1900 by Austrian biologist Karl Landsteiner. It is the classification of human blood based on the inherited properties of the red blood cells as determined by the presence or absence of antigens A and B. Antigens are foreign substances that induce immune response in the body, especially the production of antibodies. Antibodies are proteins produced in response to a specific antigen. It usually acts specifically against the foreign substance in an immune response.

Antigens A and B are usually carried on the surface of the red cells depending on the blood type. Blood group A typically have type A antigens on the surface of their red cells while blood group B have type B antigens on the surface of their red cells. If type B blood is transfused into type A, the red cells in the injected blood (type B) will be destroyed by the antibodies in the recipient’s blood. The anti-B antibodies of type A blood will recognize it as alien and burst the introduced red cells in order to cleanse the blood of the foreign substance. In the same way type A red cells will be destroyed if it is injected into a person with type B blood.

People with blood group O have neither A antigens nor B antigens on their red cells and thus are universal donors but they have antibodies A and B in their blood (plasma) so they can only receive blood from people with the same blood group (O). Individuals with blood group AB usually have antigens A and B and so they are universal recipients meaning they would not elicit any immune response against blood from blood group A, B, AB and O donors but they can only donate blood to individuals with the same blood group AB. The most common cause of death from a blood transfusion is an error in which an incompatible type of ABO blood is transfused.

Another important blood grouping system that is used to determine the compatibility of transfusion donors and recipients is the Rh blood group system. Individuals can either be positive (+) or negative (-) for the Rh blood group. Individuals with Rh+ blood group can receive blood from Rh+ or Rh- blood donors whereas individuals with Rh- blood group can only receive blood from Rh- blood donors. In emergencies, Rh- blood types are generally used for blood transfusion when there is no time to check the Rh compatibility of the blood transfusion recipient.

I hope that you’ve been able learn something new and amazing about that red substance in our human bodies called blood.